1. Field of the Invention
This invention pertains generally to solid-state electrochemical device assemblies, and more particularly to a modular parallel electrochemical cell in series stack.
2. Description of Related Art
Steadily increasing demand for power and the atmospheric build up of greenhouse and other combustion gases has spurred the development of alternative energy sources for the production of electricity. Fuel cells, for example, hold the promise of an efficient, low pollution technology for generating electricity. Because there is no combustion of fuel involved in the process, fuel cells do not create any of the pollutants that are commonly produced in the conventional generation of electricity by boilers or furnaces and steam driven turbines.
Unfortunately, the present cost of electrical energy production from fuel cells is several times higher than the cost of the same electrical production from fossil fuels. The high cost of capitalization and operation per kilowatt of electricity produced has delayed the commercial introduction of fuel cell generation systems.
A conventional fuel cell is an electrochemical device that converts chemical energy from a chemical reaction with the fuel directly into electrical energy. Electricity is generated in a fuel cell through the electrochemical reaction that occurs between a fuel (typically hydrogen produced from reformed methane) and an oxidant (typically oxygen in air). This net electrochemical reaction involves charge transfer steps that occur at the interface between the ionically conductive electrolyte membrane, the electronically-conductive electrode and the vapor phase of the fuel or oxygen. Water, heat and electricity are the only products of one type of fuel cell system designed to use hydrogen gas as fuel. Other types of fuel cells that have been developed include molten carbonate fuel cells, phosphoric acid fuel cells, alkaline fuel cells, proton exchange membrane fuel cells. Because fuel cells rely on electrochemical rather than thermomechanical processes in the conversion of fuel into electricity, the fuel cell is not limited by the Carnot efficiency experienced by conventional mechanical generators.
Solid-state electrochemical devices are normally cells that include two porous electrodes, the anode and the cathode, and a dense solid electrolyte membrane disposed between the electrodes. In the case of a typical solid oxide fuel cell, the anode is exposed to fuel and the cathode is exposed to an oxidant in separate closed systems to avoid any mixing of the fuel and oxidants due to the exothermic reactions that can take place with hydrogen fuel.
The electrolyte membrane is normally composed of a ceramic oxygen ion conductor in solid oxide fuel cell applications. In other implementations, such as gas separation devices, the solid membrane may be composed of a mixed ionic electronic conducting material (“MIEC”). The porous anode may be a layer of a ceramic, a metal or, most commonly, a ceramic-metal composite (“cermet”) that is in contact with the electrolyte membrane on the fuel side of the cell. The porous cathode is typically a layer of a mixed ionically and electronically conductive (MIEC) metal oxide or a mixture of an electronically conductive metal oxide (or MIEC metal oxide) and an ionically conductive metal oxide.
Solid oxide fuel cells normally operate at temperatures between about 900° C. and about 1000° C. to maximize the ionic conductivity of the electrolyte membrane. At appropriate temperatures the oxygen ions easily migrate through the crystal lattice of the electrolyte. However, most metals are not stable at the high operating temperatures and oxidizing environment of conventional fuel cells and become converted to brittle metal oxides. Accordingly, solid-state electrochemical devices have conventionally been constructed of heat-tolerant ceramic materials. However, these materials tend to be expensive and still have a limited life in high temperature and high oxidation conditions. In addition, the materials used must have certain chemical, thermal and physical characteristics to avoid delamination due to thermal stresses, fuel or oxidant infiltration across the electrolyte and similar problems during the production and operation of the cells.
Since each fuel cell generates a relatively small voltage, several fuel cells may be associated to increase the capacity of the system. Such arrays or stacks generally have a tubular or planar design. Planar designs typically have a planar anode-electrolyte-cathode deposited on a conductive interconnect and stacked in series. However, planar designs are generally recognized as having significant safety and reliability concerns due to the complexity of sealing of the units and manifolding a planar stack.
In addition, conventional stacks of planar fuel cells operated at the higher temperature of approximately 1000° C. have relatively thick electrolyte layers compared to the porous anode and cathode layers applied to either side of the electrolyte and provides structural support to the cell. However, in order to reduce the operating temperature to less than 800° C., the thickness of the electrolyte layer has been reduced from more than 50-500 microns to approximately 5-50 microns. The thin electrolyte layer in this configuration is not a load bearing layer. Rather, the relatively weak porous anode and cathode layers must bear the load for the cell. Stacks of planar fuel cells supported by weak anodes or cathodes may be prone to collapse under the load.
Tubular designs utilizing long porous support tubes with electrodes and electrolyte layers disposed on the support tube reduce the number of seals that are required in the system. Fuel or oxidants are directed through the channels in the tube or around the exterior of the tube. However, tubular designs provide less power density because of the relatively long current path on the electrodes since the current collection for the entire tube occurs on only a small area on the circumference of the tube. This contributes to internal resistive losses thereby limiting power density.
In addition, the concentration of the reactants often diminishes as gas flows through the channels along the length of the tubes if an insufficient volume of reactants is directed through the apparatus. Decreased gas concentration at the anode, for example, will result in a reduction in the electrical output of the cell depending on the position of the cell in the stack. Increasing the volume of fuel or oxidants flowing through the apparatus may result in excess reactants exhausting the system along with the reaction products of the electrochemical device. Excess reactants are typically burned to provide operating heat for the solid fuel cells in conventional devices. Excess reactants that exhaust the system and are burned further reduce the efficiency of the apparatus.
Another significant problem encountered with planar stacks with repeating cell elements is that the failure of one cell may result in the failure of the entire stack. Malfunctioning cells in present designs may require cooling the stack and taking it off line to replace a single cell.
Thus, present solid-state electrochemical devices incorporating conventional designs are expensive to manufacture and may suffer from safety, reliability, and/or efficiency concerns.
Accordingly, there is a need to provide a stack or array of electrochemical devices, such as solid oxide fuel cells, that are capable of operating efficiently at lower temperatures and use less expensive materials and production techniques. Stack designs that reduce the cost of materials and manufacturing while increasing the reliability of fuel cells and other solid state electrochemical devices, may allow for the commercialization of such devices that have been previously too expensive, inefficient or unreliable to exploit. The present invention satisfies these needs, as well as others, and generally overcomes the deficiencies in conventional devices.